One type of emitter-locator system in current use estimates line of bearing from two or more antenna-array receivers to the unknown emitter location, and produces a fix based on the intersection of the bearing lines. This type of system is ultimately limited in accuracy because of the superposition of modes representing multi-hop or multi-layer propagation of the radio signal to and from the ionosphere, each mode appearing at a slightly different bearing and/or time delay.
A second type of emitter locator system uses the measurement of time-difference-of-arrival (TDOA) between modes received at three or more antenna arrays to produce a hyperbolic fix (intersection of two TDOA hyperbolas) of the unknown emitter location.
A third type of emitter locator uses the measurement of vertical angle (VA) at a single receiver site along with the determination of bearing to determine radial distance along a measured line of bearing at which an emitter is located.
These second and third types of system are ultimately limited for ranges exceeding ground-wave propagation (about three hundred miles) by the accuracy to which the ionosphere is measured, that is, the effective height of each layer.